1. Field of the Invention
The present invention relates to an optical head for use in an optical disk system for reading information from an optical disk or recording information on an optical disk, and more particularly to an optical head which carries out a focusing operation by utilizing a moire pattern.
2. Description of the Prior Art
Generally, an optical head used in an optical disk system has a focusing mechanism. European Patent Publication No. 253,442, for example, discloses a focusing mechanism for detecting a focusing error by utilizing a moire pattern formed by a pair of diffraction gratings. In the disclosed mechanism, the two diffraction gratings are arranged on an optical path between an optical disk and a detector such that strips of one diffraction grating intersect those of the other at a minute angle. This arrangement of the diffraction gratings causes the moire pattern formed on the detector to rotate in accordance with divergence of the beam. Thus, a focusing error may be detecting by measuring the rotation of the moire pattern.
On the other hand, it is necessary for the optical head of the optical disk system to carry out tracking besides focusing. In the most typical known tracking method, the laser beam is split by a diffraction grating into three beams. According to this three-beam method, the diffraction grating splits the laser beam into a zero-order beam for reading information and plus and minus first-order beams for tracking purposes. The zero-order beam is irradiated on the rows of pits of the optical disk, while the first order beams are irradiated on opposite sides of the rows of pits. A tracking correction is effected by detecting a difference in the amount reflected by the optical disk between the plus and minus first-order beams. Thus, the tracking mechanism employing the three-beam method and the focusing mechanism utilizing a moire pattern both require diffraction gratings disposed on the optical path. When the two mechanisms are used in parallel, there arises the problem of interference between the diffraction grating for tracking and the diffraction gratings for focusing.
With the focusing mechanism disclosed in the above patent publication, the plurality of pit rows act as a diffraction grating when the beam does not correctly converge on the tracks of the optical disk. In this case, the pit rows may form a false moire pattern through interference with the proper diffraction gratings disposed on the optical path, which impairs a satisfactory focusing effect.